fix(frontend+mobile): ChatBotComponent QuickHelp fixes + welcomeMessage i18n fallback (Vue + Flutter)

Fixes bugs introduced by MR !146 (merged) (config-based QuickHelp):

ChatBotComponent fixes (Vue + Flutter):

• Template used $t(button.textKey) with locale map objects → SyntaxError
• serviceKey stored locale map instead of resolved string
• just-chat detection compared against missing i18n key
• Dual-prompt condition only checked hiddenPromptKey, not hiddenPrompt
• All QuickHelp tests updated to new config format

welcomeMessage fallback (all 14 locales — Vue + Flutter):

• Re-add chatbot.welcomeMessage to all 28 locale files (14 JS + 14 Dart)
• Two-tier: config overrides i18n. Config missing → i18n works.
• Prevents 'chatbot.welcomeMessage' raw key showing in chat
• Flutter: same pattern via tr('chatbot.welcomeMessage') fallback